In my game, I want to alter slightly the current game balance by making some options quite more lethal.
In particular, I want to obtain the following results (mix of general and specific situations):
a) Ambushes should be much more dangerous even when performed by clearly inferior opponents: if an adventuring party does not take the proper precautions, even a bunch of low-level goblins that staged an ambush should represent a non-trivial danger.
b) Stealthy approaches to enemies should be more lethal even for non-dedicated backstabbers: if a warrior manage to silently slip behind a standing guard, he too should be able to dispatch the guard without raising alarm, even if likely not as cleanly as a rogue.
c) Taking somebody prisoner by keeping him under aim should be a real menace for the prisoner: if a bunch of city guards surround the party keeping them under fire with their crossbows, the party should feel a greater pressure about having those crossbows aimed at them than the few hitpoints that they usually risk now.
d) Taking somebody hostage by menacing him with a weapon should be a real option even after 1st level: the standard movie and novels situation where a villain take as an hostage somebody poiting a kife at the throat, or the hero is surpried by the antagonist with a gud at the back of his head, right now is almost impossibile without relying purely on roleplay.
I think all these results can be obtained in Pathfinder without rebalancing massively everything with a couple quite simple changes to the rules, but I may be missing some unforeseen consequences, so I would gladly hear other opinions and even alternative suggestions.
My proposed changes:
1. Each time somebody lose his DEX bonus to AC due to any maneuver or check, if the margin of success/failure of the check/maneuver is 10+, any hit to the target by the source of the check is considered a coup de grace.
So, if my character is flat-footed due to a surprise check failed with a margin of 10+, if I'm hit it will be a coup-de-grace.
If I'm in battle and somebody sneak at my back with a stealth check successful by a margin of 10+, his attacks will be coups-de-grace, while the attacks by the other enemies I'm battling will be normal.Also, if somebody pin me down and succeed by 10+, he can then deliver its damage as a coup-de-grace.
2. If somebody, for whatever reason including the previous cases, let a drawn slashing or piercing melee weapon (or ranged weapon of the bow, crossbow or firearm type at a point-blank range, or spell with a touch attack that deal lethal damage) be aimed at him without being able/willing to enter combat in that round, the enemies keeping the weapon aimed at him will then be able at any moment to deliver a single coup-de-grace as an immediate action.
So, if I'm not willing to immediately start a fight with the approaching guards and let them point their crossbows at me at close range, I will hten have to be very careful about later resisting arrest or anything, as they'll be able to hit me with very dangerous coups-de-grace.
At the same time, if somebody have a knife placed at my back and is ordering me to obey him , I'll have to fear the situation much more than a simple 1d4 of hit points...The immediate considerations I've made are that "Uncanny dodge" is suddenly much more important, perception and stealth skills become fundamental, and grapple become really much more dangerous.